1. Field of Invention
The invention concerns a two-platen injection molding machine without tie bars, more particularly, an injection molding machine having a machine frame, a fixed platen, a movable platen, a drive mechanism that translates the movable platen, a device for generating clamping force on a closed mold, and a device to compensate for the deformation forces occurring during generation of the clamping force that would otherwise affect parallelism of the platens.
2. Description of the Related Art
In prior art injection molding machines, the fixed platen, also called the injection platen, is connected via tie bars in its four corner regions to an end platen, which is stationary during operation. The tie bars serve as a guide for a movable platen, which is driven relative to the fixed platen by means of a drive mechanism supported on the end platen. The tie bars together with the fixed platen and the end platen form a stable power transfer (clamp) mechanism that provides accurate guiding of the moveable platen and maintains parallelism between the mold mounting surfaces of the fixed and movable platens during mold closure. A drawback of such machines is that difficulties arise from the limited space between tie bars in the mold mounting region of the clamp mechanism that is encountered during installation or replacement of a mold. This is especially true for a large mold construction, which may have outwardly protruding hydraulic cylinders to operate sliding mold elements, etc.
To make the mold mounting region more accessible, injection molding machines without tie bars have been the focus of increasing development effort in recent years. In this type of injection molding machine, a C-shaped frame carries the aforementioned fixed platen and end platen, and generally serves to oppose the forces generated during the mold clamping function. Given the absence of tie bars to connect the platens, upward bending of the sections of the C-frame supporting the platens can occur during generation of the clamping force, so that the mold mounting surfaces of the platens are moved obliquely outward. In other words, due to this C-frame deflection when tonnage is applied to the mold, the platens and the mold elements mounted thereon are no longer parallel.
Some solutions have already been proposed in the prior art to compensate for this C-frame deflection. For example, platens are coupled to the C-frame so that they are guided independently of the frame, as by a pivoting connection. In prior art solutions, however, a relatively high cost is incurred to overcome the adverse effects. Another shortcoming of such machines is the large overall length and weight of the machines caused by the structure required to stabilize C-frame machines. For this reason, it has already been proposed in the prior art to produce two-platen injection molding machines without tie bars. A machine configured in this fashion is known, for example, from WO 94/17977, especially FIG. 7.
An injection molding machine is shown in WO 94/17977 in which the movable platen is arranged directly on the moveable end of the machine frame. Movement of the platen occurs via a hydraulic cylinder in which the piston rod is fastened on the inside of the fixed end of the machine frame, whereas the cylinder that guides the piston rod is positioned on the outside of the moving end of the frame.
The hydraulic cylinder-piston unit therefore has the dual tasks of bringing the mold halves together and applying the required clamping force to the closed mold. An additional hydraulic cylinder serves to compensate for the deformation forces occurring during generation of the clamping force that would otherwise affect parallelism of the platens. This second hydraulic cylinder is arranged beneath the drive cylinder. Both cylinders are fed separately. This variant of a two-platen injection molding machine has several shortcomings.
First, it should be mentioned that environmental protection is not adequately addressed by a purely hydraulic solution to drive the machine. A significant volume of hydraulic oil is required since both the traverse movement (demanding relatively little power) and the generation of clamping force must be produced by the same cylinder.
A second, related drawback is that a correspondingly large cylinder is required as a result of the dual tasks of the hydraulic cylinder, as described above, so that the machine weight is increased proportionately. Moreover, an unusually large amount of hydraulic fluid is moved even during traverse of the platen (which requires little power), resulting in a significant environmental hazard, given the leaks or line breaks that can occur during normal operation.
A third shortcoming of the hydraulic solution lies in the use of a hydraulic cylinder to compensate for frame deformation. Despite the relative incompressibility of hydraulic oil, hydraulic systems are inherently elastic so that a high force must be applied in the compensation cylinder to prevent deformation or keep it sufficiently small. These cylinders must therefore also be dimensioned relatively large.